Rests in Dorico are just the absence of anything else - they’re not typically something that need inputting at all.
If there was previously nothing in that gap, on the bottom stave before the first barline, the usual explanation is that MusicXML has been imported and the following note (as a byproduct) has its “Starts Voice” property turned on. Selecting that first note and unsetting that property will fix it.
If you really do want to input a rest - and again, it’s much quicker to just move the caret along the rhythmic grid and enter notes where required - you need to turn on rest input with comma, set the note value, then type any note (A-G) or Y (the generic key for inputting rhythms). This will have the desired effect of overwriting a note with a rest.
On the other hand, if there’s already a rest at that location, this is a fruitless exercise unless you also hit O to turn on Force Duration (before inputting the rest), as without Force Duration Dorico just thinks “ah well there’s already a legal (e.g. whole bar) rest at this location, and the user’s trying to add another rest. I’d better join those back together as per the global Notation Options”.